A Fly-by connection is provided as one of methods of connecting a plurality of receivers to one signal source, such as a driver, etc. The Fly-by connection is a method of connecting a plurality of receivers in a daisy chain, and is also referred to as a connection one after another, or a daisy chain connection. For example, the Fly-by connection is used for connecting a memory controller, which operates as a driver, and memories, such as a plurality of SDRAMs (Synchronous Dynamic Random Access Memories), etc., which operate as receivers.
In the case where a signal transmission speed is low, and a rising time and a falling time of the signal are long, in a configuration of connecting a plurality of receivers in a Fly-by connection, there has not been much problem with reflection at branch parts caused by signal reflection at the individual receivers. In the case where a signal transmission speed becomes high, and a rising time and a falling time become short, when a plurality of receivers are connected by a Fly-by connection, multiple reflection at branch parts occurs because of signal reflection at the individual receivers, and thus there is a problem in that reflected waves overlap with each other, and quality of a signal waveform is deteriorated.
As a countermeasure, there is a method of adjusting a line length of a signal transmission line such that reflected waves do not overlap with each other, and a method of inserting a stub resistor into a part of a daisy chain, etc. Also, there is a method of controlling impedance of a wire line of a main line part that connects from a signal source to a branch, and between individual branches, and impedance of wire lines of branch parts that connect branches and receivers. However, it is demanded to have an implementation area by any one of the methods, and it is difficult to uniquely determine impedance or a resistor value of a wire line depending on enlargement of a wiring area and a package characteristic of a receiver to user. Accordingly, it becomes difficult to apply these methods.
In a Fly-by connection, if a branch length is zero, there becomes no problem with the above-described problem of reflection. However, in a receiver package actually used, there are package line lengths of wire bonding, etc., that connects a die on which a circuit is formed and terminals of the package. Accordingly, for example, even if branch lengths are zero on a printed circuit board, branch lengths necessarily arise because of package line lengths, and overlapping of reflected waves due to multiple reflection occurs. Further, package line lengths are different depending on a receiver used. Also, package line lengths are different in accordance with a connected signal, and thus it is difficult to take a measure against reflection at branch parts on the side of the printed circuit board.
Also, there is a technique in which impedance of two transmission lines after a branch is made two times impedance of the transmission line before the branch in order to match impedance so that influence of signal reflection is suppressed (for example, refer to Japanese Laid-open Patent Publication No. 2004-222092). However, this technique is suitable for a star-connection configuration, but is difficult for applying to a configuration in which a large number of receivers are connected to one signal source in a Fly-by connection. For example, in the case of applying the technique to a configuration in which four receivers are connected to one signal source in a Fly-by connection, impedance of the transmission lines of the branch parts of the receiver becomes two times, four times, eight times, and 16 times, which are unattainable impedance values on a printed circuit board having a same layer configuration.